By virtue of excellent magnetic properties, R—Fe—B permanent magnets as typified by Nd—Fe—B systems find an ever increasing range of application. For modern electronic equipment having magnets built therein including computer-related equipment, hard disk drives, CD players, DVD players, and mobile phones, there are continuing demands for weight and size reduction, better performance, and energy saving. Under the circumstances, R—Fe—B magnets, and among others, high-performance R—Fe—B sintered magnets must clear the requirements of compact size and reduced thickness. In fact, there is an increasing demand for magnets of compact size or reduced thickness as demonstrated by a magnet body with a specific surface area (S/V) in excess of 6 mm−1.
To process an R—Fe—B sintered magnet of compact size or thin type to a practical shape so that it may be mounted in a magnetic circuit, a sintered magnet in compacted and sintered block form must be machined. For the machining purpose, outer blade cutters, inner blade cutters, surface machines, centerless grinding machines, lapping machines and the like are utilized.
However, it is known that when an R—Fe—B sintered magnet is machined by any of the above-described machines, magnetic properties become degraded as the size of a magnet body becomes smaller. This is presumably because the machining deprives the magnet surface of the grain boundary surface structure that is necessary for the magnet to develop a high coercive force. Making investigations on the coercive force in proximity to the surface of R—Fe—B sintered magnets, the inventors found that when the influence of residual strain by machining is minimized by carefully controlling the machining rate, the average thickness of an affected layer on the machined surface becomes approximately equal to the average crystal grain size as determined from the grain size distribution profile against the area fraction. In addition, the inventors proposed a magnet material wherein the crystal grain size is controlled to 5 μm or less during the magnet preparing process in order to mitigate the degradation of magnetic properties (JP-A 2004-281492). In fact, the degradation of magnetic properties can be suppressed to 15% or less even in the case of a minute magnet piece having S/V in excess of 6 mm−1. However, the progress of the machining technology has made it possible to produce a magnet body having S/V in excess of 30 mm−1, which gives rise to a problem that the degradation of magnetic properties exceeds 15%.
The inventors also found a method for tailoring a sintered magnet body machined to a small size, by melting only the grain boundary phase, and diffusing it over the machined surface for restoring magnetic properties of surface particles (JP-A 2004-281493). The magnet body tailored by this method still has the problem that corrosion resistance is poor when its S/V is in excess of 30 mm−1.
Methods for the preparation of R—Fe—B magnet powder for bonded magnets include the hydrogenation-disproportionation-desorption-recombination (HDDR) process. The HDDR process involves heat treating in a hydrogen atmosphere to induce disproportionation reaction on the R2Fe14B compound as the primary phase for decomposing into RH2, Fe, and Fe2B, and reducing the hydrogen partial pressure for dehydrogenation to induce recombination into the original R2Fe14B compound. When a magnet powder is prepared by the HDDR process, it consists of crystal grains with a size of about 200 nm which is smaller than the grain size in sintered magnets by one or more order, and particles of degraded properties present at the magnet surface in a magnet powder with a size of 150 μm (S/V=40) account for only 1% by volume at most. Then no noticeable degradation of properties is observable. By controlling the disproportionation and recombination reactions in the HDDR process, grain refinement can be achieved while inheriting the crystal orientation of the original R2Fe14B grains. Then a so-called anisotropic powder can be prepared. The anisotropic powder has the advantage of very high magnetic properties, as compared with isotropic powder prepared by the melt quenching process. However, bonded magnets prepared therefrom have a maximum energy product of about 17 to 25 MGOe, which value is as low as one-half or less the maximum energy product of sintered magnets.
For R—Fe—B magnets, it is known to add Dy or Tb as part of R to enhance the heat resistance. The intrinsic coercive force is also increased by the addition. However, the HDDR process is not applicable to those alloys containing certain amounts of Dy and Tb because Dy and Tb act to inhibit disproportionation reaction in hydrogen.
It was thus believed difficult in a substantial sense to produce an R—Fe—B ultrafine magnet body having excellent magnetic properties and heat resistance and free of degradation of magnetic properties.